Image-guided percutaneous tumor ablations are widely practiced to treat cancers such as liver and kidney cancers. Those therapies include cryoablation, radiofrequency ablation (RFA), microwave ablations (MWA), laser ablations, and irreversible electroporation (IRE). Some imaging modalities that are useful include ultrasound imaging, computed tomography (CT), and magnetic resonance imaging (MRI).
While ultrasound and CT are often imaging modality of choice for guiding probes into the target lesion and monitoring the therapeutic effect, MRI can be preferable as ionizing radiation is not used and a tumor may not present distinctive contrast against the surrounding tissue in the other modalities. Intraprocedural MRI can provide high-resolution 2- or 3-dimensional images with superior soft tissue contrast. Furthermore, MRI offers unique advantage in monitoring of thermal effects during ablation therapies: in cryoablation, formation of the “ice ball” can be monitored as signal void on MR images; in RFA, MWA, and laser ablations, MR thermometry based on proton resonance frequency (PRF) shift method enables monitoring the thermal dose in the tissue. The combination of MRI's capabilities to delineate the tumor and monitor the thermal effect enables physicians to ensure the sufficient ablation margin during the procedures, and hence it potentially leads to reduce tumor recurrence. In addition, MRI does not expose the patient and the clinical staffs to ionizing radiation.
Despite those advantages in target localization and treatment monitoring, the MRI-guidance imposes a technical challenges; a typical MR gantry does not give physicians an easy access to the patient at the isocenter, where images are acquired. Therefore, patient needs to be moved out from the gantry for the probe placement, and moved back into the gantry for imaging. This move-in-and-out approach inhibits the physicians from visually checking the probe location with respect to the target while inserting it unlike other imaging modalities such as ultrasound and CT fluoroscopy. One solution is to treat the patient in an open-configuration scanner. Open-configuration scanners allow physicians access the patient during the scan. However, open-configuration scanners have not become a mainstream, because of the limited imaging capability compared to the conventional closed-bore scanners.
Another solution is to guide the probe using a probe-guide device. The probe-guide device can maintain the probe trajectory mechanically outside the MRI scanner. Therefore, the probe can be inserted into the target as long as the target location remains unchanged. Such devices can be categorized as either “table-, gantry-, and floor-mounted” systems or “patient-mounted” systems depending on how the devices are fixed. The patient-mounted systems are directly attached to the patient's skin rather than the room (or the image coordinate system), hence one can hypothesize that the systems are less vulnerable to the motion of the patient than the other category of the devices. Furthermore, the patient-mounted systems can achieve probe guidance with relatively simple mechanism, because they are designed to be mounted at the probe entry point on the skin, and only need a mechanism to angulate the probe about the entry point.
Thus, there is still a need to accurately place the device on the patient relative to the treatment area. Further, even with a patient-mounted system, patient motion still must be accounted for. Both the body surface and the internal organ can move independently, and cause displacements of the device and the target lesion. Given that imaging for planning and probe placement takes place at different time points in the operation, there is a need to keep track of the displacement of the device due to the motion of the body throughout the operation.
Despite the fact that fiducial markers are an integral tool used to facilitate, automated methods for registering images or assisting in therapeutic planning using the fiducial markers are still limited and error prone when used to provide accurate placement and patient motion. It would therefore be desirable to provide a system and method for automating image analysis and image registration that does not suffer from the drawbacks described above.